A motor vehicle typically has a bumper bar supported on a body of the motor vehicle by energy absorbers which convert into work a fraction of the kinetic energy of a low speed impact on the bumper bar. Such bumper energy absorbers commonly include relatively moveable structural elements attached to the body and the bumper bar and a resistance medium between the structural elements. In a high speed impact, kinetic energy is converted into work by plastic deformation of body structure of the motor vehicle between a passenger compartment thereof and the point of impact. As motor vehicles have become more compact, however, the energy absorbing capability of the body structure of the motor vehicle has decreased as the span between the passenger compartment and the bumper bar has decreased. A bumper energy absorber described in U.S. Pat. No. 5,370,429 supports a bumper bar close to a body of a motor vehicle except when sensors on the vehicle detect an impending impact. In that circumstance, the bumper energy absorber extends the bump bar out from the body to increase the span between the passenger compartment and the bumper bar and thereby increase the fraction of the kinetic energy of a high speed impact on the bumper bar that is converted into work. A bumper energy absorber according to this invention is a novel alternative to the bumper energy absorber described in the aforesaid U.S. Pat. No. 5,370,429.